If media is retired and deleted from a Deduplication Folder using the Backup Exec user interface, that media will show up again after the Backup Exec services are restarted. When an inventory operation is run, Backup Exec ends up with multiple copies of the media it its database.
Retiring and deleting the media removes the reference to the virtual media in the Deduplication Folder from Backup Exec's database. During the service restart, Backup Exec scans the Deduplication Folder and finds references to all the media that was retired and deleted. However, this is a partial media record because the media itself has not been read. This is similar to the barcode scan that takes place on a physical tape library when BE services start up. This media is listed with a Media Description of (Unknown Media).
When an inventory operation takes place, BE reads the contents of the media in the Deduplication Storage Folder and reads the media GUID from the virtual media. This GUID doesn't match the GUID of the media that was created by the startup scan. At this point, the media from the startup scan is moved to Offline Media, and a new record representing the true contents of that position of the Deduplication Folder is created. And thus, BE ends up with two media records with the same media label (e.g. OST00000001-ABCDEF1234567890).
The extra media listed in the Offline Media node of the Media tab does not represent actual media in the deduplication storage folder. And, it does not appear to cause any problems with BE functionality. It does however clutter up the media views. It is safe to go into the Offline Media node and delete OST media that is listed as (Unknown Media).
This issue is now resolved in Backup Exec 2010 R3 Hotfix 159965